In at least one known CT system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
To reduce the total scan time, a "helical" scan may be performed. To perform a "helical" scan, the patient is moved while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a one fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed.
Reconstruction algorithms for helical scanning typically use helical weighting ("HW") algorithms which weight the collected data as a function of view angle and detector channel index. Specifically, prior to filtered back projection, the data is weighted according to a helical weighting factor, which is a function of both the view angle and detector angle. As with underscan weighting, in a HW algorithm, projection data is filtered, weighted, and backprojected to generate each image.
In CT fluoroscopic systems ("CT Fluoro"), data collected from a helical scan may be utilized to generate sequential frames of images to help, for example, in guiding a needle to a desired location within a patient. A frame, like a view, corresponds to a two dimensional slice taken through the imaged object. Particularly, projection data is processed at a frame rate to construct an image frame of the object.
With known CT Fluoro systems, the general objective is to increase the frame rate while minimizing image degradation. Increasing the frame rate provides many advantages including, for example, that an operator physician is provided with more timely (or more up to date) information regarding the location of a biopsy needle. Typically, however, increasing the frame rate is at odds with minimizing image degradation. For example, the more often projection data is filtered, weighted and backprojected, the slower the frame rate. The frame rate is thus limited to the computational capabilities of the CT Fluoro system.
It would be desirable to provide a CT Fluoro system that has an increased frame rate without significantly adversely affecting image quality.